Sometimes a post or page is updated frequently for some reason. Perhaps it contains daily deals, site notifications or other things like that. The update subscription plugin I built allows individual users to subscribe to updates on a piece of content, confirm their subscription and receive notifications every time the content is updated. The users can also unsubscribe from the notifications at any time.
Jason came to me a few years ago looking for a way to track the popularity and correctness of comments on his site. I developed a solution that was great for him and his users and it has lasted over 3 years now without modification.